1. Field of the Invention
The present invention relates to an image forming apparatus that carries out image formation on a recording medium such as paper using an electrophotographic method and to a control method therefor.
2. Description of the Related Art
In an image forming apparatus, such as a digital copying machine, image formation is carried out by scanning a photosensitive drum with modulated laser light to form an electrostatic latent image, developing the electrostatic latent image using toner, and transferring the toner image to a recording medium such as paper.
For example, there are apparatuses that determine the timing at which to start the scanning of the photosensitive drum with the laser light based on a detection result of a means (a reflective-type sensor) that detects a leading edge position of a sheet being conveyed as the recording medium. An image forming apparatus has been proposed that transfers the toner image after carrying out control to determine the timing so that a leading edge of the image and a recording position on the sheet are aligned at a transfer position facing the photosensitive drum and thereby avoids missing the image due to the image being displaced on the sheet (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H09-244339).
There is also an image forming apparatus including a leading edge detecting sensor which, aside from the object of aligning the recording position at the transfer position, detects the timing in which the leading edge of the sheet arrives on a conveying path and changes the sheet conveying speed and/or the conveying path.
An image forming apparatus that uses an actuator-type sensor instead of a reflective-type sensor as the means for detecting the leading edge position of the sheet being conveyed has also been proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H09-077309).
According to the conventional art described above, a reflective-type sensor or an actuator-type sensor is used in an image forming apparatus as the leading edge detecting sensor that detects the leading edge position of the sheet being conveyed. In addition, in Japanese Laid-Open Patent Publication (Kokai) No. H09-244339, processing is started when the leading edge detecting sensor detects that the leading edge of a sheet has arrived. For this reason, a method is used where an output of the leading edge detecting sensor is connected to an interrupt terminal of a central processing unit (hereinafter referred to as “the CPU”) and the leading edge of the sheet is detected when an interrupt has occurred from the leading edge detecting sensor.
However, as shown in FIG. 9, the leading edge of a sheet P conveyed on a conveying path while being sandwiched by conveying rollers 813 in the image forming apparatus constantly meanders on the conveying path as the sheet advances. Accordingly, when the leading edge of the sheet P is detected by a reflective-type leading edge detecting sensor 801, for example, the output signal of the leading edge detecting sensor 801 fluctuates such as a wave as shown by reference numeral 802 and only becomes stable over time. Accordingly, there is the following problem for a method where the output signal of the leading edge detecting sensor 801 is inputted to the interrupt terminal of the CPU to directly produce an interrupt request for the CPU. That is, since a plurality of interrupt requests are issued at every transition point, it is not possible to correctly determine the leading edge positions of respective sheets or the number of sheets that have passed the sensor.
For this reason, normally, as shown by reference numeral 803, after the output signal of the leading edge detecting sensor 801 becomes the transition point, interrupts are inhibited from occurring until the output signal has stabilized for a predetermined time period. That is, a method is used where a noise removing counter is installed between the leading edge detecting sensor 801 and the CPU (hereinafter referred to as “the first example”). However, due to the noise removing counter being installed, a delay AD is produced between detection time of the leading edge of the sheet P and the issuing time of the actual interrupt request. The delay AD varies due to the type of sheet P, the conveying path, and conditions relating to the conveying rollers 813 and the like, and cannot be predicted in advance.
As an alternative to the method described above, there is a method that periodically monitors the output of the leading edge detecting sensor 801 using a control section and detects a change in the output of the leading edge detecting sensor 801 as a sheet leading edge (or a sheet trailing end of a sheet) (hereinafter referred to as “the second example”). In this case also, a detection error “Δd” expressed byΔd≦V*t is produced where “V” represents the sheet conveying speed and “t” represents a sensor detection period of the control section.
In addition, since it is necessary to carry out processing using a timer or the like to somewhat eliminate detection errors due to noise as described above, the detection error Δd is further increased.
If uncorrected, the delay ΔD and the detection error Δd described above can cause misalignment when transferring an image onto the sheet. On the other hand, since this sort of image forming apparatuses have been increasingly used as portable printers in recent years, there is demand for improved accuracy for the print position. That is, to prevent displacement of transferred images and/or to prevent fluctuations in the transfer positions of images, it is necessary to minimize the delay ΔD or the detection error Δd.
For this reason, especially for a high-speed image forming apparatus where the sheet conveying speed V is high, for the first example described above, it is necessary to shorten the count operation of the noise removing counter (i.e., to lower the INT level in FIG. 9). For the second example, the sensor detecting period t of the control section needs to be made as short as possible.
However, if the count operation of the noise removing counter is shortened, this can cause erroneous operations due to noise included in the output signal of the leading edge detecting sensor. Also, to shorten the sensor detecting period t of the control section, a control section that operates at high speed is required, which can lead to an increase in power consumption and in more radiation energy being radiated to the periphery of the image forming apparatus.
There is also the following problem. FIG. 10A shows the case where the sheet P is conveyed in a normal state. FIG. 10B shows the case where the sheet P is conveyed with the sheet P skewed with respect to the conveying direction, resulting in the possibility of part of the image being printed outside the sheet P so that information is lost. To prevent this, it is necessary to correctly detect a skew amount (tilt amount) of the sheet and thereby carry out skew correction for the image, write timing correction for the image, and the like.
When detecting the correct skewing amount of the sheet, there is a method that disposes a plurality of leading edge skewing detecting sensors as shown in FIGS. 11A to 11C. However, according to this method, by merely having one of the leading edge skewing detecting sensors detect the sheet leading edge, it is not possible to determine whether the sheet P is skewed (see FIG. 11A) or whether the width of the sheet P is narrow (see FIG. 11B). Also, even if the sheet P is skewed, the skewing amount will only be known when the sheet P is conveyed further and the second leading edge skewing detecting sensor detects the sheet P (see FIG. 11C).
For this reason, to correct the image write timing when scanning the photosensitive drum with the laser light to form the latent image, the position at which skewing of the sheet is detected needs to be a sufficient distance upstream of a position corresponding to an image write start position. Since this results in an increase in the length of the conveying path required to detect skewing of the sheet, there is the problem that the image forming apparatus becomes large.